Change `--template` to allow use of extensionless templates.

The intent is to allow bash process substitution: e.g.,
`--template <(echo "foo")`.

Previously pandoc *always* added an extension based on the
output format, which caused problems with the absolute filenames
used by bash process substitution (e.g. `/dev/fd/11`).

Now, if the template has no extension, pandoc will first
try to find it without the extension, and then add the
extension if it can't be found.

So, in general, extensionless templates can now be used.
But this has been implemented in a way that should not cause
problems for existing uses, unless you are using a template
`NAME.FORMAT` but happen to have an extensionless file `NAME` in
the template search path.

Closes #5270.

1 files changed, 10 insertions, 8 deletions

diff --git a/src/Text/Pandoc/App/OutputSettings.hs b/src/Text/Pandoc/App/OutputSettings.hs
index f8c27bee4..38cd35455 100644
--- a/src/Text/Pandoc/App/OutputSettings.hs
+++ b/src/Text/Pandoc/App/OutputSettings.hs
@@ -25,7 +25,7 @@ import qualified Data.Text as T
 import Text.DocTemplates (toVal, Context(..), Val(..))
 import qualified Control.Exception as E
 import Control.Monad
-import Control.Monad.Except (throwError)
+import Control.Monad.Except (throwError, catchError)
 import Control.Monad.Trans
 import Data.Char (toLower)
 import Data.List (find)
@@ -111,13 +111,15 @@ optToOutputSettings scriptingEngine opts = do
           _ | not standalone -> return Nothing
           Nothing -> Just <$> getDefault
           Just tp -> do
-            -- strip off extensions
-            let tp' = case takeExtension tp of
-                        "" -> tp <.> T.unpack format
-                        _  -> tp
-            getTemplate tp'
-              >>= runWithPartials . compileTemplate tp'
-              >>= fmap Just . templateOrThrow
+            let getAndCompile fp =
+                   getTemplate fp >>= runWithPartials . compileTemplate fp >>=
+                      fmap Just . templateOrThrow
+            catchError
+              (getAndCompile tp)
+              (\e ->
+                  if null (takeExtension tp)
+                     then getAndCompile (tp <.> T.unpack format)
+                     else throwError e)
 
   (writer, writerExts, mtemplate) <-
     if "lua" `T.isSuffixOf` format